I. Field of the Invention
The present invention relates to the production of tubes and, more particularly, to a method of, and a mill for, pilger rolling of tubes.
In modern practice cold rolling of tubes is carried out mainly on rolling mills equipped with movable stands. There is a tendency to constructionally improve mills for cold rolling of tubes by increasing the area of deformation, employing rolls which are smaller in size, increasing the operating speed of such rolls, making use of the multi-strand and so-called warm rolling, and improving the sizing tool and the procedure for preheating an initial tubular billet to be rolled.
An increase in the number of passes effected by a roll mill stand is possible to achieve by reducing the weight of such stand and by balancing the masses which perform reciprocated motion. It should be observed, however, that the reduction in weight of the working stand results in lower rigidity of the system comprised of a roll, ball bearing, chock and roll housing, thereby adversely affecting the dimensional accuracy of the tubes, as well as the quality of their surfaces.
Moreover, the reduced weight of the working stand fails to have any appreciable effect on the production capacity of the rolling mills, since the insufficient rigidity of such working stands makes it necessary to lower the rolling force.
II. Description of the Prior Art
A certain increase in the number of double passes of a working stand has been achieved by using various types of balance arrangements, for example, pneumatic, counterweight and spring balance arrangements. The application of such arrangements for balancing movable masses renders the rolling mill more complicated in construction and larger in size, and imposes more stringent requirements on the manufacturing accuracy and mounting technique of the fast-operating units of the rolling mill.
To enhance the production efficiency of the mills for cold rolling of tubes, such mills have been equipped with roll mill stands adapted for double-strand rolling. These stands are heavy in weight and, therefore, are capable of performing but a limited number of passes, since they require a bulky balance arrangement for their most effective operation.
Rolling mills for cold rolling of tubes effected by means of rollers have found wide application in the production of cold-rolled tubes having superthin walls. In these mills, as distinct from the mills where cold rolling of tubes is effected by means of rolls, the drawing of a tube is performed over a cylindrical mandrel by means of rollers forming a pass of constant radius equal to the radius of the finished tube.
When rolling is effected by means of rollers, the travelling distance of the working stand extends for a length of 1.8 to 2.0 times the rotating length of the roller, i.e. for a length greater than that of the tube section being rolled. This, in turn, results in an appreciable dynamic force transmitted to a drive means. The total drawing of tubes on cold rolling mills effected by means of rollers rarely achieves triple value, since the tube wall is reduced mainly in thickness and partially in diameter.
It has been found in the course of operation of the rolling mills furnished with rollers that an increase in the rate of feeding, as well as in total deformation effected in a single pass, results in a lower quality of the tube surface and in an impaired accuracy of their geometrical dimensions, which is manifested in the imperfect roundness and nonuniform thickness of the tube wall.
It has been found that the quality of the surface and the dimensional accuracy of the tubes cold-rolled at the tube-rolling mills are improved as the rigidity of the working stands and rolls thereof is increased, and the production efficiency of such mills is enhanced as the weight of the mill reciprocatingly moving parts is lowered. The reduction in weight of the mobile stand, however, results in its lower rigidity, which, in turn, makes it impossible to ensure improved drawing and higher accuracy of the tube dimensions. British Pat. No. 1,149,822 describes a rolling mill for intermittent cold rolling of tubes effected by means of undriven rolls. This rolling mill comprises a stationary mounted working stand which accommodates two rolls. The mandrel rod in the working stand is rigidly connected with a gripping jaw, the gripping jaw being connected by means of a spring to a carriage. The carriage driving mechanism in this rolling mill is made in the form of a barrel cam fitted with a loop-like groove. Received in said groove is a lug rigidly fixed on the carriage body. The mandrel used is cylindrical in shape. A tubular billet is fed by means of a lead screw which interacts with the housing of the carriage driving mechanism, having the barrel fixed therein. During the feeding operation the body of said driving mechanism is moved towards the working stand. The same lead screw is used to withdraw the rod.
The rolling mill of the British patent referred to above is unsuitable for rolling of tubes to be reduced 1.5 to 3 times in inside diameter. In this case it will be necessary to replace the barrel when resetting the mill. Furthermore, the tube feeding mechanism should be moved together with the billet being fed, which adds to dynamic loads acting on the drive means. Since the mill in question is not provided with a mechanism for turning the finished tube, it is impossible to use one mandrel for rolling two tubes, since the finished tube is not fixed in the jaw and interacts only with the working rolls, the mandrel and with the incoming billet.